1. Field of the Invention
The present invention is directed to methods of manufacturing cups and containers made from expandable thermoplastic materials that provide improved physical properties, especially regarding the strength and non-leaking characteristics of the cups and containers.
2. Description of the Prior Art
Containers used for liquid or solid foodstuffs (e.g., drinking cups, containers for frozen confections and the like) are required to be substantially leak-proof so that the filled containers may be handled reliably during shipping and/or consumer usage with minimal risk of the contents leaking and thereby creating an inconvenient mess or bodily harm. In this regard, containers made from expandable thermoplastics particles (beads of expandable polystyrene or EPS, for example), sometimes referred to as foam cups, have a base molded to a generally cylindrical side wall and are susceptible to leakage when proper fusion of expandable particles is not achieved.
Often, the rim of a cup or container is adapted to accept a lid, which is snapped or press fit over the top of the rim of the container, securing the lid to the rim. When the strength of the container wall and rim is insufficient, the act of grasping the cup or container with a hand can cause the cup or container wall and rim to deflect and change shape causing the lid to be unintentionally removed from the cup or container. This situation provides an opportunity for the contents of the cup or container to be spilled thereby creating an inconvenient mess or bodily harm.
U.S. Pat. No. 4,191,803 discloses expandable particles of a styrene polymer containing 3 to 8% by weight of a blowing agent, having a particle size of 0.8 to 0.25 mm in diameter and pre-expanded particles that have a coating that includes an ester of sucrose on the surface of the pre-expanded particles. In the disclosure, it is explained that when EPS having a small particle size is used as a material for a thin walled food container, the thickness corresponds to 2 to 3 expanded particles. In this case, the possibility of leakage becomes an important problem. In contrast, leakage seldom becomes a problem in the case of an article formed with EPS particles having larger particle size used for containers having a thicker wall, where the thickness corresponds to 4 to 8 expanded EPS particles.
U.S. Pat. No. 2,857,341 discloses a method of preparing EPS particles for molding by forming the raw particles to include uniformly dispersed fatty acids in the particles and seeping the particles, with a particle size of less than 10 mesh in a blowing agent.
U.S. Pat. No. 3,461,088 discloses a method of producing EPS particles that includes pre-expanding EPS particles that contain a blowing agent and coating the pre-expanded particles with a water insoluble organic compound selected from paraffins, chloroparaffins, and fatty acids.
U.S. Pat. No. 3,647,723 discloses a process for preparing EPS particles that includes forming the raw polystyrene particles in the presence of a wax and a metal salt of a fatty acid and adding a blowing agent to the particles.
U.S. Pat. No. 3,663,466 discloses EPS particles that contain polystyrene, a blowing agent and a salt of a fatty acid uniformly dispersed in the particle.
U.S. Pat. No. 3,736,273 discloses a method of producing EPS particles that includes polymerizing styrene in the presence of a metal salt of an aliphatic carboxylic acid and adding a blowing agent to the particles.
U.S. Pat. No. 4,036,675 discloses a disposable cup or container and closure made from EPS, which is lined on one or both sides with unoriented polyolefin film. The film is secured to the foamed plastic base material using as a heat-sensitive adhesive a vinylic polymer or polyamide resin.
U.S. Pat. No. 4,698,367 discloses EPS particles that include polystyrene resin and a blowing agent in which a copolymer composed of a fluorinated vinyl polymer part and a hydrophilic vinyl polymer part covers or is included on the surface or in the surface layer of the EPS particles.
U.S. Pat. Nos. 4,703,065 and 4,720,429 disclose EPS cups molded from EPS particles coated on at least a portion of the surface of the particles with a fluorosurfactant.
U.S. Pat. No. 7,294,655 discloses EPS particles, used to form cups or containers. The EPS particles are coated with a composition that includes a liquid part that includes a liquid polyethylene glycol and a solid part that includes a polyolefin wax, a metal salt of higher fatty acids, and optionally a solid polyethylene glycol and/or a fatty bisamide or fatty amide.
U.S. Pat. Nos. 4,798,749 and 4,840,759 disclose cups molded from EPS particles that include isopentane as all or part of the blowing agent and coated with 300 ppm of silicone oil and 1000 ppm zinc stearate.
Thus, EPS that is conventionally used to mold low-density, thick-walled cups and containers is typically coated with approximately 1000 ppm of zinc stearate and approximately 250 ppm of silicone oil. Using higher levels of zinc stearate to make conventional low-density, thick-walled cups and containers results in poor fusion, which results in defective cups and the problem of lids being unintentionally removed from the cup or container as described above.
However, when high-density, thin-walled cups or containers are desired, the typical levels of lubricant used to make conventional low-density, thick-walled cups and containers results in poor leak resistance and the problem of lids being unintentionally removed from the cup or container as described above.
There is an unmet need in the art for methods to produce cups and containers that overcome the above-described problems.